Tsubasa is really really good, and you dont need to know the other series. They just play cameos, though I do also recommend xxxHolic. They have the Tsubasa episodes subbed over at Live-Evil, so if you wanna watch an episode to get an idea of what it is, try it. It's got a good mix of action and love, and cross-references a lot of Clamp manga.

I'm personally more into romance/shoujo series myself... so I really dont know what to recommend aside from those. My personal favorites are currently Othello, The Wallflower, and Couple, all with funny stories and very pretty art.

I you have a local bookstore that sells manga, I recommend just going there and picking out a few volumes to read in the store. That's what I do, so I wont buy something I dislike.